1. Technical Field
Embodiments of the present invention relate to a mirror buried under an active semiconductor area of a semiconductor component such as a diode.
2. Discussion of the Related Art
FIGS. 1A and 1B are respectively a cross-section view and a top view of a metal-semiconductor-metal type photodiode MSM such as described in U.S. Pat. No. 5,525,828, which is incorporated by reference. The photodiode comprises an active semiconductor area 1 on which are formed two metal electrodes E1 and E2 preferably having an interdigited structure. Electrodes E1 and E2 are separated by an insulating area 2. This photodiode comprises a reflective element 3 such as a Bragg mirror placed under active area 1 above a substrate 4. An example of a Bragg mirror is a stacking of alternate silicon oxide and silicon nitride layers.
Each electrode E1 and E2 corresponds to the anode of a Schottky diode formed of an electrode E1 or E2 and of semiconductor area 1. The photodiode is thus equivalent to two Schottky diodes head-to-tail.
A voltage is applied between electrodes E1 and-E2. In this example, electrode E1 is grounded and electrode E2 is at a positive voltage equal to 10 volts. The Schottky diodes having electrode E1 as an anode are reverse-biased. Accordingly, a space charge area forms under insulating area 2 between electrodes E1 and E2. The electric field induced by this space charge area is shown on FIG. 1A as a set of oriented curves running from electrode E2 at 10 V to grounded electrode E1.
Insulating area 2 is transparent to let through the photons of an incident light beam. These photons are absorbed by active semiconductor area 1 and “electron-hole” pairs are created. The electrons and the holes are accelerated by the electric field present in the space charge area, then collected by one of electrodes E1 and E2. An electric current is created between electrode E2 and electrode E1.
The photons which have not been absorbed by the active area before reaching the level of reflective mirror 3 are reflected towards the surface of active area 1. These photons can then be absorbed as they propagate back up. Because the maximum penetration depth of a photon into silicon is relatively high, approximately 17 microns, the use of a reflective mirror 3 enables “concentrating” the photons close to electrodes E1 and E2 where the electric field is high. This enables having more sensitive photodiodes exhibiting a higher operation frequency.
The use of Bragg mirrors, however, requires implementing complex manufacturing methods. One of the known methods comprises the forming of a stacking of silicon oxide and nitride layers on a support substrate, then the gluing of a silicon wafer on this stacking, followed by the planning of the silicon wafer to obtain an active area having a thickness of a few hundreds of nanometers. Furthermore, for a Bragg mirror to be reflective, very accurate silicon oxide and nitride thicknesses must be provided, which does not simplify their manufacturing. Furthermore, the Bragg mirror is typically reflective for a single wavelength value.